Fluorinated ion exchange polymers having carboxylate functional groups, and optionally sulfonate functional groups as well, are known in the art. One principal use of such polymers is as a component of a membrane used to separate the anode and cathode compartments of an electrochemical cell such as a chloralkali electrolysis cell. Such membrane can be in the form of a reinforced or unreinforced film or laminar structure. Fabrication of such membrane is ordinarily carried out with polymer in melt fabricable form, i.e., wherein the carboxylic groups are --COOR functional groups where R is C.sub.1 to C.sub.8 alkyl, and, if present, sulfonic groups are in --SO.sub.2 X form where X is F or Cl.
A carboxylic ester group which is attached to a carbon atom which in turn has one or more fluorine atoms attached thereto is, however, subject to easy hydrolysis by water in either the liquid or gaseous state to a free carboxylic acid group. Partial hydrolysis readily occurs during washing of polymer for the purpose of purifying it, during quenching of polymer pellets of film in an aqueous quench bath following extrusion of the polymer, and during storage of polymer in any physical form by moisture in the atmosphere.
It is undesirable for a polymer of the carboxylic type to have part of the carboxylic groups in the form of free carboxylic acid groups during melt fabrication thereof, such as extrusion into film and fabrication of laminar structures therefrom, e.g. lamination to films having sulfonyl halide functional groups or to reinforcing fabrics, for several reasons. First, inasmuch as a polymer with carboxylic acid groups has a higher melt viscosity at a given temperature than the corresponding polymer with carboxylic ester groups, in the interest of batch-to-batch uniformity of polymer to be melt fabricated into film, so that more uniform extrusion conditions can be used, conversion of any variable content of carboxylic acid to ester is desirable. Second, inasmuch as hydrolysis of carboxylic ester polymer probably occurs non-uniformly, i.e., to a great extent on the surface of cubes, pellets, film, etc., while to a much lower extent in the interior portions, such material even within a single batch is of non-uniform character or composition from point to point, and therefore will not extrude as uniformly from point to point as will a polymer which is all in the ester form. Third, at sufficiently high temperature, carboxylic acid groups will decarboxylate, which results in loss of some of the original ion exchange capability, i.e., a change to higher equivalent weight. Fourth, in fabrication of laminar structures from films, the higher melt viscosity of polymer having some ester groups hydrolyzed to acid groups interferes with good melt flow of the polymer during lamination, and thus leads to lower adhesion of the layers laminated to one another.
Additionally, it would be desirable to have an efficient process for esterifying carboxylic polymer which is substantially completely hydrolyzed, e.g. following use in an ion-exchange application, or after any intentional hydrolysis, as a first step in recovery of such polymer for reprocessing and reuse.
Reesterification of hydrolyzed carboxylic ester groups by the conventional technique of treating with an alcohol, with or without an acid catalyst, is, however, slow. It is also incomplete, inasmuch as water is a by-product of the esterification, and water leads to further hydrolysis; put another way, this reaction attains only an equilibrium between the ester and acid groups. Additionally, slow diffusion of by-product water out of the polymer through the newly esterified surface layer slows down this process.
It is therefore an object of this invention to provide an improved process, which is both rapid and substantially complete, for esterifying fluorinated polymers having carboxylic acid groups to polymer having carboxylic ester groups.